Avian leukosis viruses (ALVs) which have defined genetic compositions will be used to examine the role of viral envelope antigens and the c region of viral RNA in ALV-associated disease. Viruses which are present in ALV-associated lymphomas will be characterized to determine the relationship of tumor associated viruses to infecting viruses Novel tumor associated viruses will be identified by examining tumors for novel viral proteins and RNAs. A major effort will be made to culture tumor associated viruses which code for novel proteins and RNAs. Novel viruses will be tested for their oncogenic potential. Our working hypotheses are: (1) any ALV which actively replicates in a target tissue has the potential of becoming a transforming agent for that tissue. (2) ALVs become transforming agents when they recombine with cellular genes to give viral genes which affect the growth and/or differentiation of tissues. (3) Transforming agents which are formed during ALV infections cause disease in specific target cells after short latent periods.